1. Field of the Invention
The present invention relates generally to interface circuits and more particularly to a monitor interface circuit to be connected to a multi-scan display monitor which can display display data derived from various types of personal computers.
2. Description of the Prior Art
Different types of personal computers are adapted to generate digital display data of various modes, such as for example, display data mode in which 8 colors are displayed by the combinations of display data of 3 bits corresponding to three primary colors represented as R (red), G (green) and B (blue), a display data mode in which 16 colors are displayed by the combinations of display data of the above 3 bits (R, G, B) and an additional display data of 1 bit (I) for gray scale, a display data mode in which 64 colors are displayed by the combinations of respective colors each being formed of 2 bits, a display data mode in which a single color is displayed (so-called green monitor mode), and so on.
Display data, on the other hand, have various kinds of horizontal scanning frequencies, such as 15.734 kHz of the NTSC system, 21.8 kHz, 18.4 kHz, 30.12 kHz or the like.
Known display monitors have been produced as special types of display monitors in correspondence with the display modes of display data that the personal computers generate. So, when a user exchanges his personal computer for a different type of personal computer having different display data, the user is also required at the same time to exchange the display monitor. This is a disadvantage since a color display monitor is very expensive.
A multi-scan display monitor has been proposed in which the horizontal scanning frequency of an incoming display data signal is automatically discriminated and then the deflection device of the monitor is controlled in correspondence with the horizontal scanning frequency of the incoming display data signal. By using this multiscan display monitor, incoming display data signals of different horizontal scanning frequencies can be reproduced satisfactorily.
However, a monitor interface circuit to be connected to such a display monitor has not yet been proposed which can form predetermined video or display signals from incoming display data signals having different display modes.